Hydrofoil technology dates back to the late 1800's and was expanded upon in the 1950's. Consequently, there exist a wide variety of hydrofoils designed to lift the hull of a vessel out of the water when the vessel is moving in the forward direction. The purpose of hydrofoils has generally been to lift the hull of a vessel out of the water when at speed, known as being “foil-bound,” minimizing the amount of surface area in contact with the water (the wetted surface area) and therefore minimizing drag and consequently increasing speed as well as fuel efficiency.
The operation of hydrofoils is generally known to those skilled in the art as operating under the same principles as an air foil: by moving a cambered foil through a liquid, a pressure differential is created, resulting in a lift force orthogonal to the direction of movement. In other words, as a cambered foil moves in the forward direction through a medium, a resultant upward or downward force is applied to the foil, depending on the shape of the camber.
Affixing hydrofoils to motor-powered vessels has generally been more popular than affixing hydrofoils to wind-powered vessels. Some reasons for this are that motor-driven propulsion provides (relatively) high speed, quick acceleration, and a single-direction, constant force. The foil designs may therefore be smaller and need only account for a single force vector—the motor-driven propeller.
As for wind-powered vessels, the wind speed and direction can often be unpredictable. Additionally, those skilled in the art are also aware that water vessels powered by wind are often subjected to a heeling moment. A heeling moment is a moment induced by the wind against the sailing rig causing the vessel to tilt or heel to one side. Such heeling makes it difficult for a vessel to be lifted out of the water by vertically-lifting hydrofoils fixed to the hull, as the lift is no longer vertically out of the water, but more in a sideways direction.
Some designs, as in U.S. Pat. No. 6,499,419 to Bussard, have attempted to extend the hydrofoils in several directions such that there is a portion of the foil that will provide vertical lift at certain angles of heel. Such “surface piercing” designs by necessity extend out beyond the hull of the vessel. The widening of the foils to accommodate for the heeling enlarges the vessel's footprint significantly and likely becomes difficult to handling. Such designs do little to counter the heeling moment, causing difficulty in controlling the vessel. In addition, it is generally known in the art that such surface piercing hydrofoils are not ideal in anything other than smooth water, as they are subject to the effects of wave action, resulting in an uncomfortable ride for the passengers.
Another problem caused by heel is a loss of power from the sailing rig. As the vessel and the attached sailing rig are blown by the wind, they tilt away from the wind. As a result, the effective surface area of the sail is reduced, thereby reducing the capability of the sailing rig to extract the maximum amount of power from the available wind. In non-hydrofoil sailing vessels, a keel is often used to counter this heeling moment and to maintain the vessel and sailing rig as vertical as possible. As a result, the effective surface area of the sail is increased and thereby the power derived from the wind. Keels, however, are necessarily heavy, increase the draft of the boat, and are not ideal for a vessel having hydrofoils that lift it out of the water.
One solution to avoid any heeling is to have another sort of counterweight force, as in the International Moth-class hydrofoil boat design and some windsurfer designs, such as U.S. Pat. No. 5,471,942 to Miller et al. In the case of a windsurfer, the passenger serves as the counterweight by holding onto the boom of the sailing rig (flexibly affixed to the board) and leaning away from the sail while in the case of the boat (having a mast rigidly affixed to the boat), the passenger(s) “hike-out” onto an extended deck/platform and are attached to a harness and trapeze. These designs typically have one forward and one aft t-foil that are used to provide the lift. Although fast, these designs are highly unstable and are highly reliant upon the weight and skill of the passenger(s) to serve as the counterweight and controllers. Significantly relying on such a counterweight tends to make operating the vessel difficult, uncomfortable, and generally limits the size/displacement of the vessel to an amount that would allow such counterweight to remain effective while foil-born.
Other designs have used sponsons, outriggers, or multiple hulls for a broader beam to counteract the heeling moment with buoyancy. Better known examples of these are, for example, trimarans such as the WindRider Rave, the Hobie Trifoiler, and the French-Swiss l'Hydroptère, which, for the purposes of this discussion, are sufficiently similar to U.S. Pat. No. 5,054,410 to Scarborough. In these multi-hull designs, the additional hulls serve the purpose of countering the moment induced by the wind onto the sailing rig. As a result, the vessel remains substantially upright with respect to the water surface. The hydrofoils that are employed in these designs may therefore be fully submerged. Although these designs cause the sail to remain fully upright (perpendicular to the water surface) and thereby maximizing the available wind, the wide beam, by design, creates a wide vessel, resulting in, for example, difficulty in storing, transporting, and navigating in narrow passages.
Sailing vessels with fixed hydrofoil designs in general are also limited from traveling in shallow water. Similar to sailboats with fixed keels, in such shallow water environments, the chances of running aground or hitting underwater structures such as a reef are increased. Challenges relating to the storage of a vessel having fixed hydrofoils are similar to those with a large beam.
Having adjustable hydrofoils in powerboats may also improve the stability and efficiency of the vessel in certain conditions, particularly during high-speed turns when the centrifugal forces tends to cause the vessel to tip to the outside of the turn. Adjustable hydrofoils may counteract this tipping.
Accordingly, a need exists for a hydrofoil design that may be fitted to a monohull water vessel that is capable of providing sufficient stability and weight-carrying capacity (particularly when the design is scaled to hull sizes large enough to carry two or more passengers). An additional need is the ability for the hydrofoils to be retractable to allow for easy storage, transport, and shallow water navigation.